Use of compound-material wafers, in particular, silicon on insulator (SOI) type wafers, as starting materials in the semiconductor device manufacturing process is becoming more and more important. One way to fabricate such wafers with good crystalline quality at reasonable cost is by transferring a layer from a donor substrate also of good crystalline quality onto a handle substrate. Specifically, donor and handle substrates are bonded, and then the donor substrate is detached at a predetermined splitting area previously formed in the donor substrate. In the SMART-CUT® process, the splitting area is formed by implanting atomic species, in particular hydrogen ions or rare gas ions, into the donor substrate. The advantage of this process is that the remainder of the donor substrate, which remains after the transfer of the layer onto the handle substrate, can be re-used as a donor substrate in a subsequent component material wafer fabrication process. Thus, one donor substrate can serve for several compound-material wafers, like SOI type wafers.
It appeared, however, that the donor substrate could only be re-used three to four times, because from run to run, the crystalline quality of the donor substrate deteriorated. As a consequence, the crystalline quality of the final SOI substrate also deteriorated.
Efforts have been made to allow the recycling of the used donor substrates. Japanese patent application JP19980114176 (Japanese publication JP11297583), for example, proposes to first polish the donor substrate to remove the surface step at the edge of the wafer, which is present after the transfer of a layer onto the handle substrate, and then to carry out a second finishing polishing step before re-using the remainder of the donor substrate as a new donor substrate. U.S. Pat. No. 6,211,041, although not being directed to recycling processes, discloses a different approach, which consists in providing, right from the beginning, a silicon substrate with an appropriate oxygen content. An appropriate oxygen content is claimed to prevent the creation of crystal defects, which according to the authors, are related to the presence of oxygen precipitates generated in the SOI device layer during the heat treatments that the wafers undergo during the SOI fabrication process.
Despite this disclosure, however, the number of possible re-uses of donors substrate is still not satisfactory. In particular, it appears that controlling the initial oxygen content does not prevent crystal defects from appearing in the final SOI layer after multiple reuse. Thus, improvements in this area are needed and these are now provided by the present invention.